1. Field of the Invention
The present invention relates to lithium secondary batteries and methods of manufacturing the batteries.
2. Description of Related Art
Mobile information terminal devices such as mobile telephones, notebook computers, and PDAs have become smaller and lighter at a rapid pace in recent years. This has led to a demand for higher capacity batteries as the drive power source for the mobile information terminal devices. With their high energy density and high capacity, lithium-ion batteries, which perform charge and discharge by transferring lithium ions between the positive and negative electrodes, have been widely used as a driving power source for such mobile information terminal devices.
The mobile information terminal devices tend to have higher power consumption as the functions of the devices, such as moving picture playing functions and gaming functions, increase. It is strongly desired that the lithium-ion batteries that are the drive power source for the devices have further higher capacities and higher performance in order to achieve longer battery life and improved output power. In view of these circumstances, attempts have been made to use as the negative electrode active material an aluminum alloy, a silicon alloy and a tin alloy, which can absorb a large amount of lithium per unit volume, so that the discharge capacity of the lithium secondary battery can be increased.
In the lithium secondary battery that employs the just-described negative electrode active material, however, the negative electrode active material pulverizes or peels off from the negative electrode current collector because the negative electrode active material undergoes a great volumetric change during occlusion and release of lithium. This causes breakage of the current collection structure in the negative electrode. As a consequence, electron conductivity within the negative electrode degrades, and the cycle performance becomes poor.
In view of the problem, Japanese Published Unexamined Patent Application No. 2002-260637 discloses a negative electrode that achieves good cycle performance. This negative electrode is formed by sintering a negative electrode active material layer containing a polyimide binder and a negative electrode active material composed of a material containing silicon under a non-oxidizing atmosphere.
In addition, PCT Publication No. WO2004/004031 and Japanese Published Unexamined Patent Application No. 2007-242405 disclose that the cycle performance can be further improved by varying the type of the polyimide of the negative electrode.
Nevertheless, even with the use of the just-mentioned conventional polyimide resin or the like as the binder, the adhesive strength and the strength of the resin are still insufficient. Consequently, the problem still remains that the current collection performance in the negative electrode degrades (especially, the current collection performance after the initial charge/discharge cycle degrades) and improvements in the initial charge-discharge efficiency, the discharge rate performance, and the cycle performance are insufficient.
Accordingly, it is a primary object of the present invention to provide a lithium secondary battery and a method of manufacturing the battery that can inhibit degradation of the current collection performance in the negative electrode (particularly the degradation of the current collection performance after the initial charge/discharge cycle) and improve the initial charge-discharge efficiency, the discharge rate performance, and the cycle performance.